A typical active impedance line feed circuit includes tip and ring amplifier circuits being controlled to exhibit predetermined a.c. impedance and d.c. feed resistance characteristics for the purpose of supplying energizing current for a two wire communication line. These characteristics are determined by respective a.c. and d.c. feedback networks associated with an amplifier circuit within the line feed circuit. In one example of the line feed circuit, application Ser. No. 386,871, a diode is combined with the d.c. feedback network to limit the direct energizing current in the case of a short and hence low resistance communication line. Another feature of the exemplified line feed circuits of the previously mentioned application is that through selective biasing the tip and ring amplifiers are caused to remain in linear operation throughout OFF HOOK and ON HOOK states of the communication line. This has the advantage of conveniently permitting a.c. signal testing of the communication line via the line feed circuit during the ON HOOK state. However this biasing has the disadvantage of making it appear that the communication line is energized from a battery of lesser potential than is actually the case. In this respect the exemplified line feed circuits of the previously mentioned applications are at a disadvantage when compared to a purely resistive feed as exemplified by V. V. Korsky in U.S. Pat. No. 4,103,112 issued July 25, 1978 to the present assignee. The maximum length and hence resistance of the communication line to which a specified minimum energizing current can be supplied is less than in the case of the Korsky line circuit.
In the most recent of the previously mentioned patent applications, a line feed circuit is exemplified wherein the a.c. and d.c. feedback networks are directly connected across the tip amplifier whereby circuit economies accrue.
In each of the active impedance line feed circuits of the previous applications, a problem remains as to providing at least a minimum energizing direct current for operation of a communication line of a maximum permissible resistance and at the same time providing for ON HOOK a.c. signal testing of the communication line.